I should have run
ifdown eth0 && ifup eth0, but I just ran
Can I connect to my instance and run
ifup eth0? What should I do?
Once you have shut down the Ethernet interface the only way to bring it back online is either explicitly by running ifup on the console or rebooting the instance.
There is no remote way to bring up an interface that has been shut down.
(Assuming this is a single if device, which seems fair given if another up if existed you would already have connected via it and run if up.)